What to do if you have a leaking overflow pipe

An overflow pipe dripping outside your home is easy to ignore — but it shouldn’t be. Beyond the wasted water adding to your bills, a persistent overflow can signal a fault in your plumbing or heating system that gets worse and more expensive the longer it’s left. Here at Plus Heat, here’s what to check and what to do.

What Is an Overflow Pipe?

Overflow pipes are safety outlets built into water-holding components around your home. When water levels rise too high — due to a faulty valve, excessive pressure, or a stuck float — the overflow pipe diverts that water outside rather than letting it flood internally.

The following components all have overflow pipes:

• Toilet cistern

• Cold water storage tank (loft)

• Central heating feed and expansion tank (loft)

• Hot water cylinder

• Combination boiler (pressure relief valve)

• Baths, basins, and sinks

How to Identify Which Pipe Is Leaking

Overflow Pipe Near the Toilet or Bathroom

If the dripping pipe exits at ground or first-floor level through the wall behind or near your toilet, it’s almost certainly your toilet cistern overflow.

The most common cause is a faulty float valve — the mechanism that tells the cistern to stop filling once it reaches the correct level. If the float is set too high, damaged, or has developed a hole, it won’t shut off the water supply properly and the cistern will overflow.

What to check:

• Look inside the cistern — is the water visibly above the overflow level?

• Try gently bending the float arm slightly downward to lower the water level

• If the float itself has a hole or feels heavy with water inside, it needs replacing

• If adjusting the float doesn’t help, the washer on the float valve may need replacing — shut off the water supply to the cistern first before attempting this

Replacement float valves are inexpensive and available at any plumbers’ merchant or DIY store. If you’re not confident replacing it yourself, a plumber can sort it quickly.

Boiler Overflow Pipe (Pressure Relief Valve)

Combination boilers don’t have a traditional overflow pipe — what they have is a pressure relief valve (PRV). This is a safety device that releases water when the system pressure exceeds its safe limit, typically 3 bar. The PRV pipe is usually copper and runs from the boiler through an outside wall, often terminating close to ground level.

If water is dripping or running from this pipe, your boiler pressure is likely too high, the PRV itself may be faulty, or there may be an underlying issue with the expansion vessel.

Do not attempt to fix this yourself. A dripping PRV is a sign your system pressure isn’t being managed correctly, and repeatedly ignoring it can lead to boiler damage or a more serious pressure failure. Call a Gas Safe registered engineer to diagnose and fix it safely.

Also worth knowing: don’t confuse the copper PRV pipe with the condensate pipe, which is plastic and carries waste water from the boiler to a drain — these are two completely separate pipes with different functions.

Overflow Pipe From the Eaves or Loft

If the dripping pipe exits high up on the property — from the eaves or through the wall near the roofline — it’s coming from a tank in your loft. This will be either your cold water storage tank or your central heating feed and expansion tank.

If it overflows when the heating is on: the water level in the central heating feed tank is likely set too high when cold, causing it to expand beyond capacity when the system heats up. The float valve may need adjusting downward.

If it overflows when heating hasn’t been used: the ball float valve may be allowing a slow trickle of water in continuously, which only becomes noticeable when demand isn’t masking it. Check whether the float is stuck in a raised position or the valve seat needs a new washer.

In both cases, look inside the tank. If the water level is clearly above the overflow inlet, adjust the float arm or replace the valve. If the tank is cracked or the overflow happens constantly regardless of float position, call a plumber.

Is a Leaking Overflow Pipe Dangerous?

It depends on the source. A toilet cistern overflow is a nuisance and a waste of water, but it’s not an immediate safety risk. A dripping boiler PRV is a different matter — it indicates your heating system is operating above safe pressure thresholds and needs professional attention promptly.

In any case, leaving any overflow unresolved risks:

• Structural water damage to walls, ceilings, or foundations

• Mould growth in affected areas

• Increased water bills from continuous waste

• Damage to your boiler or heating components if pressure issues go unchecked

Frequently Asked Questions

Why is there a pipe dripping water outside my house?

It’s an overflow pipe. The most common sources are a toilet cistern with a faulty float valve, a loft water tank with the water level set too high, or a boiler pressure relief valve releasing excess pressure. Identifying which pipe it is — by its location, material, and height on the property — will point you to the cause.

Why is hot water coming out of my overflow pipe?

Hot water discharging from a copper pipe near your boiler almost certainly means your pressure relief valve is activating. This happens when boiler pressure exceeds 3 bar. It may be caused by a failing expansion vessel, an overfilled system, or a faulty PRV itself. Switch the boiler off and call a Gas Safe engineer — don’t ignore this one.

Can I fix an overflowing toilet cistern myself?

In many cases, yes. Adjusting the float arm or replacing a worn float valve washer are jobs most homeowners can manage with basic tools and a shut-off valve. If the whole float valve assembly needs replacing and you’re unsure, a plumber can resolve it quickly and inexpensively.

How much does it cost to fix a leaking overflow pipe in the UK?

A toilet float valve replacement typically costs £50–£120 including labour. A boiler PRV replacement or expansion vessel repair can range from £150–£400 depending on the fault. With a Plus Heat cover plan, plumbing and boiler call-outs are covered — so you’re not hit with unexpected bills when something like this crops up.

Not sure which pipe is the problem — or not confident fixing it yourself? Plus Heat offers home cover plans from just 25p a day, covering boiler, central heating, plumbing, electrical, and drainage. Call our team on 0808 164 2892 and we’ll get it sorted.

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